1. Field of the Invention
This invention pertains generally to optical shutters with bistable rotary drive actuators and shutters that move a blade to a given position when power is removed. More specifically, the invention (1) describes a monostable shutter with magnetic blade return using a bistable actuator and (2) permits the use of a compact bistable coil for continuous use while preventing coil burn out.
2. Description of the Related Art
Optical shutters use an actuator to drive a blocking element, “blade”, between a first position that blocks a beam of radiation from passing through a designated open area (called an aperture) which serves as, or forms a pathway for the beam (or “beam path”) and a second position that unblocks the beam of radiation, allowing the beam to traverse the beam path through the aperture. The radiation can be any form of electromagnetic radiation, such as ultra-violet, visible or infrared radiation. The aperture can be in a frame that is directly or indirectly coupled to the actuator
The actuator can be electromagnetically activated (an “electromagnetic actuator”) so that it responds to applied electrical power to move the blade between the open and closed position. An electromagnetic actuator can be a linear solenoid, a rotary solenoid, or a brushed or brushless commutated motor.
Monostable solenoid shutters comprise a coil of wire that provides a magnetic field when electrical power is applied. The magnetic field applies a force to pulls a soft magnetic core in a given direction. Monostable shutters with soft magnetic cores need a spring to returns the core to an original position when power is removed from the core. Monostable shutters arc useful in applications that require the blade to be in a given position when power is removed. Monostable shutters have the disadvantage that they require continuous power to remain in the electrically driven state.
Bistable shutters are stable in both the open and closed states when power is removed. Bistable shutters can be created using geared motor drives that lock in a given position when unpowered. An over-center spring can be used to create a locking force in either of the two positions. The soft magnetic core of a monostable solenoid can be replaced with a hard magnet that adheres to soft magnetic material in each of the two positions to create a bistable shutter. For example, the rotary drive solenoids (RDS) produced by CVI Melles-Griot are an example of rotary solenoids with a permanent magnet core that is bistable (ref Proc. of SPIE, Vol. 6542, “Advanced electro-mechanical micro-shutters for thermal infrared night vision imaging and applications; Durfee et al). These bistable shutters can have the advantage that the blade position changes with short pulses of voltage to drive the blade between positions. Because bistable shutters are pulsed intermittently, the coils are smaller and lighter to dissipate low amounts of energy over time. Such smaller coils burn out if subjected to continuous power.
Prior art examples that are or might be related to the technology and/or purposes of the instant invention include: (1) U.S. Pat. No. 4,868,695 issued to Quatro et al. for a “Head/Arm Lock Mechanism for a Disk Drive” (1989) describing a head/arm lock mechanism including a pawl mounted to the armature of a bistable solenoid; (2) U.S. Pat. No. 5,155,522 issued to Castor et al. for “Self Centering Bi-Directional Electromagnetic Actuator” (1992) describing a system for electromagnetically activating the shutter of a camera to provide different aperture openings; (3) U.S. Pat. No. 5,159,382 issued to Lee et al. for a “Device and Method for Electromagnetically Activating the Shutter of a Camera” (1992) describing a device and method for electromagnetically activating the shutter of a camera to provide different aperture openings; (4) U.S. Pat. No. 5,497,093 issued to Sundeen et al. for a “Method and Apparatus for Diagnosing a Twin-Coil, Bi-Stable, Magnetically Latched Solenoid” (1996) describing diagnosing the electrical and mechanical operation of a bi-stable magnetically latchning solenoid by monitoring induced voltage across one of a pair of solenoid pairs not being energized; (5) U.S. Pat. No. 5,883,557 issued to Pawlak et al. for a “Magnetically Latching Solenoid Apparatus” (1999) describing a magnetically latching solenoid apparatus characterized by a non-magnetic armature carrying a permanent magnet having poles aligned with the throw axis of the device; and (6) U.S. Pat. No. 7,701,691 issued to Brundisini et al. for a “Control Device for Driving AC Solenoids and DC Bistable Solenoids, Specially for Electrovalves of Irrigation Systems” (2010) describing a control device to drive both AC solenoids and DC bistable solenoids. However, while the foregoing art examples and/or disclosures reveal a variety of forms and systems, none feature the unique combination of elements and advantages offered by the instant invention. More particularly, none disclose, anticipate or obviate a method to convert bistable shutters in monostable applications with blade return in the unpowered state, nor do any disclose, anticipate or obviate how bistable shutters with compact coils that burn out under continuous power can be used in monostable shutters.